As a conductive polymer for various applications, a conductive sulfonated polyaniline polymer has been known.
As a method for producing a sulfonated polyaniline, for example, Patent Literature 1 discloses a method for producing an aniline copolymer, the method including copolymerizing alkoxy group-substituted aminobenzenesulfonic acid with at least one compound selected from the group consisting of aniline, N-alkylaniline, and phenylene diamines.
However, a copolymer obtained by the method described in Patent Literature 1 has a low rate of sulfonation and a low solubility in water alone, and therefore causes a problem of poor workability. In addition, the copolymer has a problem of low conductivity due to difficulty in purification and the presence of impurities.
In order to overcome these problems, as a polyaniline in which an acidic group is introduced in all the aromatic rings and a method for producing such a polyaniline, Patent Literatures 2 and 3 each disclose a soluble conductive aniline polymer and a method for producing such a soluble conductive aniline polymer. The method includes: dissolving an acidic group-substituted aniline such as a sulfonic acid group-substituted aniline or a carboxy group-substituted aniline in a solution containing a basic compound; and adding dropwise an oxidizing agent to the solution to polymerize the aniline.
Contrary to the conventional established theory that it is difficult to polymerize anilines alone having a sulfonic acid group or a carboxy group, the method enables production of a high molecular mass polymer. In addition, the obtained conductive polymer exhibits excellent solubility in both acidic and alkaline aqueous solutions. Therefore, a conductive polymer having an advantage in terms of processing can be produced relatively easily from an inexpensive starting material.
Patent Literature 4 discloses a method capable of improving conductivity and solubility. In the method, an acidic group-substituted aniline such as a sulfonic acid group-substituted aniline or a carboxy group-substituted aniline is polymerized using an oxidizing agent in a mixed solution of a basic compound and a water-soluble organic solvent, and the resultant polymer is subjected to acid treatment to be improved in conductivity and solubility.
Patent Literature 5 discloses a method of oxidation polymerization. In the method, an acidic group-substituted aniline such as a sulfonic acid group-substituted aniline and/or a carboxy group-substituted aniline is dissolved in a solution containing a basic compound. The resulting solution is then added dropwise to an oxidizing agent to make a reaction system in which the molar amount of the oxidizing agent is equal to or more than that of the aniline, to perform oxidization polymerization.
Patent Literature 6 discloses a method for purifying a solution of polyaniline derivative in coexistence with a dopant of impurities by dialysis or ultrafiltration.
However, according to the methods of Patent Literatures 2 and 3, the pH in a system at the initial stage of polymerization is basic. As a result, a side reaction such as conversion of acidic group-substituted aniline as a starting material monomer to an azo compound is not sufficiently suppressed. Therefore, a byproduct contained in the polymer prevents the improvement of conductivity.
The methods of Patent Literatures 4 to 6 suppress a side reaction such as conversion of acidic group-substituted aniline as a starting material monomer to an azo compound. However, impurities cannot be removed sufficiently during separation of polymer from a polymerization reaction solution by filtration. Therefore, the resultant polymer cannot necessarily satisfy conductive performance as compared with the other conductive polymers.
The conductivity (σ) of a conductive polymer generally depends on the number (η) of carriers, the charge (q) of the carriers, and the mobility (μ) of the carriers between and in molecular chains.
In the case of a soluble conductive aniline polymer, the charge (q) of carriers is a characteristic value determined by the type of the carriers. Therefore, increasing both the number (η) and mobility (μ) of the carriers is essential for improvement of the conductivity. Increase of the molecular mass of the polymer, increase of the molecular mass of the polymer by removal of an unreacted monomer, an oligomer as a byproduct, and impurities, and the like are considered to be effective for the increase of the mobility (μ).
For example, Patent Literatures 7 and 8 disclose that an unreacted monomer and a low molecular mass substance are removed from a soluble conductive aniline polymer to improve the conductivity.